Cannot parse Cookie header in Ruby on Rails

Yesterday I resolved a client emergency for a Ruby on Rails site that continues to leave me scratching my head, even with follow-up investigation. In short, the emergency came up after an email marketing campaign was sent out in the morning, and resulted in server (HTTP 500 Status Code) errors for every customer that clicked on the email links. Despite the fact that Rails exception emails are sent to the client and me, the errors were never reaching the exception email code, so I was unaware of the emergency until the client contacted me. Upon jumping on the server, I saw this in the production log repeatedly: ArgumentError (cannot parse Cookie header: invalid %-encoding (...)): ArgumentError (cannot parse Cookie header: invalid %-encoding (...)): ArgumentError (cannot parse Cookie header: invalid %-encoding (...)): The URLs that the production log was complaining about ha...